Manual quality profiles (source/1080p/720p/480p) in the player overflow
menu plus an honest/preparatory Auto contract: adaptive when the HLS
manifest exposes multiple levels, single-rendition automatic selection
otherwise.
- server: transcodeQualityProfiles service and qualityMode session option
- player: quality menu, persisted preference, live switch with rollback
- tests: npm test script and unit tests for quality profiles
